Axcelis Technologies, assignee of the present invention, designs and sells products for treatment of workpieces such as silicon wafers during integrated circuit fabrication. Ion implanters create an ion beam that modifies the physical properties of workpieces such as silicon wafers that are placed into the ion beam. This process can be used, for example, to dope the silicon from which the untreated wafer is made to change the properties of the semiconductor material. Controlled use of masking with resist materials prior to ion implantation as well as layering of different dopant patterns within the wafer produce an integrated circuit for use in one of a myriad of applications.
Ion implanters typically include an ion source including an ionization chamber for ionizing a neutral gas from gaseous feed material or from vapor generated from a solid or liquid feed material to produce a collection of charged particles i.e. ions and electrons, hereinafter referred to as a plasma. Ions are extracted from the ion source plasma by creating an electric field between the source plasma ions and an electrode, or a series of electrodes, to form an ion beam. The beam travels through specially shaped apertures in each of the electrodes. Typical beam extraction systems are either 3 or 4 electrode systems, although other variations are possible, including very extended systems where final beam deceleration is accomplished in the proximity of the wafer. A standard three electrode system typically comprises a first electrode, the so-called plasma electrode, or arc slit electrode, or ionization chamber electrode, which may be electrically biased at the same large positive voltage as the ion source. This potential determines the beam energy. A second electrode, the so-called suppression electrode, is at a moderate negative voltage to prevent electrons from streaming back to the source chamber. A third and final electrode, the so-called ground electrode, is at ground potential. The extracting electric field is determined by the potential difference between the ionization chamber electrode and the suppression electrode, the details of their shapes, and the distance therebetween. In many ion implantation processes, it is necessary to generate beams of very different energies and different species for different doping properties, for example 5 kV boron for source-drain extensions and 80 kV arsenic for punchthrough stop. The strength of the electric fields generated by the electrodes must be tuned and adjusted to match the desired extracted beam current and maintain good ion beam optics. This process is called perveance matching. One of the most common methods for achieving good perveance matching is to move the suppression and ground electrodes relative to the ion source, more specifically, the ionization chamber electrode. The distance between the ionization chamber electrode, which defines the ionization chamber aperature, and the suppression electrode, which defines the suppression aperture, is called the extraction gap.
Advanced integrated chip manufacturing methods have brought to light various issues involving the implantation of ions at low energies. Transporting low energy ion beams is problematic because space charges within the ion beam cause the beam profile to diverge, commonly known as beam blow up. Beam blow up results in beam current loss and degradation of beam emittance. One solution that has been proposed to solve the problems associated with low energy ion implantation is molecular-ion beam implantation, wherein ionized molecules of ions are transported and implanted into a work piece as opposed to the implantation of atomic species. However, extraction of molecular ion beams require large extraction gaps as compared to more typical high current ion implantation systems. When the gap is large, as for example, comparable in size to the extraction electrodes and/or to the distance from the ionization aperture to the surrounding ground, the electric field lines tend to bend in undesirable ways that cause the beam to over or under focus and result in a loss of beam current due to clipping on apertures further down the beam line. These optics problems are exacerbated by the relative geometric length of the apertures. Due to the length of the apertures relative to other scale lengths, electric fields can become non-uniform over the length of the apertures. These focusing problems are more acute when the long dimension of the aperture is in the dispersive plane of the ion beam, such as can be found in so-called parallel to point optics systems utilized in some ion implanters.